The present invention relates to crystal growth furnaces, and, in particular, relates to radio frequency heating of the crystal growth furnaces.
Radio frequency (RF) heating has long been used as a convenient, efficient method of directly or indirectly heating materials in a furnace. The technique depends on the fact that an electrically conducting material will have currents induced within it in the presence of an RF field. These currents will generate heat within the conductor by Ohms law. The amount of power generated will depend on the product of the square of the induced current (a function of the resistivity of the material and the magnetic field strength produced in the material by the RF field) and the resistivity of the material over the induced current paths. The efficiency of current (and therefore heat) production depends on the electrical properties of the material, the frequency of the RF field, and geometric factors depending on the shape and proximity of the RF source. These geometric factors are of importance since they can be adjusted such that power is coupled only into the areas required and not into other conductors in the vicinity (e.g., a crystal growth chamber).
Typical configurations in the materials processing and crystal growth areas are cylindrical. A helical coil (water cooled to carry off ohmic heat within the coil) is powered by an RF generator. Frequencies used are typically between 0.4 and 10 Mhz, and the output currents to the coils of hundreds of amps are common. The cylindrical geometry concentrates fields inwards toward the material to be heated. Although in some applications conducting materials are directly coupled to the RF field, it is more common to heat an external conductor being called a susceptor and transferring heat to the charge by thermal radiation or conduction.
Especially at low frequencies, the RF wavelength is much longer than the total length of the coil. The currents in the coil generate axial magnetic fields with the frequency of the RF field. These in turn generate circumferential currents in the charge or susceptor. Typical susceptor configurations are cylindrical tubes, either open or closed on the ends. A special purpose configuration is a susceptor designed as a crucible or crucible holder. This is used to contain liquid materials, typically for crystal growth.
The prior technique for obtaining precise control over temperature gradients used resistance heated furnaces with multiple power supplies and control loops.
Control of the temperature gradients in the RF environment can be difficult. Some control can be achieved by the position of the susceptor or the charge in the coil. Helical coils in which the pitch of the helix varies have been used to increase coupling in one part of the susceptor or charge in another. Design of such coils is not simple since the magnetic fields produced by each turn of the coil are vector additive and are constantly changing in direction and magnitude. Finally, some control of the temperature gradients produced may be achieved by appropriate use of thermal radiation shields which selectively allow heat to escape from various parts of the working area. These shields must in general be made of electrically non-conducting materials or they too will couple to the RF field.
Two techniques which require very careful control of temperature gradients involve directional solidification of melts to produce single crystals. The Bridgman technique, FIG. 1, ideally operates in a temperature environment in which two relatively long constant temperature zones are separated by a short region in which the temperature varies linearly between the two temperatures. The hotter (upper) zone is held at a temperature somewhat above the melting point of the material to be grown and the cooler (lower) zone somewhat below the melting point. The melt is confined in a crucible and slowly lowered in this thermal environment. In a nearly planar position in the short zone in which the temperature varies, corresponding to the melting point of the material, crystallization of the melt occurs. If conditions are appropriate the material will grow as a single crystal. This may be promoted by including a seed at the bottom of the crucible which is not completely melted, or by having a sharp point or reentrant area at the bottom of the crucible which promotes the formation of only a single seed nucleus.
Gradient freeze growth, FIG. 2, is a similar technique for directional solidification and crystal growth. In this technique a rather small linear gradient is maintained across the ampoule containing the melt (hotter at the top than at the bottom). The absolute temperature at the bottom is adjusted so that all of the material is melted except for possibly a section of single crystal seed. The temperature at some point in the system is then controlled and linearly decreased with time while the gradient is maintained. The plane containing the melting point of the material moves upward in the work area and thus the crystal solidifies from the bottom up as in the Bridgman system, FIG. 2. In this technique there is no physical movement of the components. The position of the plane containing the melting point is controlled by the power input to the system.
Control of the thermal gradients in an RF heated environment such as the above has traditionally involved placement of heat and radiation shielding to make some area in the environment lose heat more rapidly than others or control of geometric factors such as the position of the susceptor in the coil, the geometry of the coil or the position of the material to be heated within the susceptor. Modification of the thermal environment to attain precise thermal conditions is tedious and basically a trial and error method, and the end results of changes in these parameters are not easily predictable thus wasting numerous hours, materials and energy.
Clearly, there is a need to be able to obtain precise thermal control when using RF fields to provide heat.